For increased performance, it is often desired to reduce transit time of electrons in N-type metal oxide semiconductor (NMOS) device channel regions and of positive charged holes in P-type MOS device (PMOS) channel regions used in a complementary metal oxide semiconductor (CMOS) devices on a substrate (e.g., integrated circuit (IC) transistor, etc. on a semiconductor substrate). Reduction in channel lengths is a favored way of reducing transit times however because such reductions induce short channel effects, multi-gate devices have been developed in which the channel regions are a portion of a non-planar semiconductor body, or “fin” covered by a gate stack. For such multi-gate devices, the transistor can be gated by the gate stack through a sidewall, as well as a top surface of the fin for better gate control.
With the improved gate control possible with multi-gate designs, the dimension of the fin may be scaled to such a point that the contact to the fin results in a parasitic resistance, Rexternal, which can severely limit the operational performance of the multi-gate device. One method of reducing the overall resistance is to dope the fin source/drain regions. For instance, a dopant may be implanted in the source/drain regions and an anneal may be carried out to activate and diffuse the dopant towards the channel region.
Where an implant and diffusion method is used, the ability to control the dopant concentration and location within the fin is limited. Furthermore, the size of other parts of a MOS device, such as the presence of a spacer around the fin, can also greatly hinder reductions in Rexternal.
Furthermore, because fin structures are free of a surrounding substrate, strain-inducing mobility enhancement techniques which have proved advantageous in the past for planar devices may not be readily adaptable to multi-gate devices. Without the ability to enhance channel mobility via strain (e.g., uniaxial or biaxial), the performance improvement in multi-gate devices resulting from the smaller channel lengths possible would be at least partially offset by a comparatively lower channel mobility. Accordingly, improved methods and structures are needed to overcome these limitations in the source/drain fin regions.
It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate corresponding or analogous elements.